1. Field of the Invention
The present invention relates to a commutating type DC circuit breaker arrangement in which a commutating circuit is improved which closes a commutating switch after interruption of a vacuum circuit breaker and modifies a DC arcing current into an alternating waveform via the commutation to complete the interruption.
2. Description of Related Art
In a circuit of a commutating type DC circuit breaker disclosed in JP-A-3-67429 (1991) which corresponds to EP-A2-0411663, a vacuum circuit breaker is connected in series to a DC circuit and a commutating circuit composed of a capacitor, a saturation reactor and a commutating switch is connected in parallel with the vacuum circuit breaker. In order to interrupt a DC current by making use of the commutating circuit, the commutating switch is adapted to be closed after a trip command permitting an exciting current to flow through a trip coil is generated from a control circuit for the vacuum circuit breaker. When one of the electrodes of the vacuum circuit breaker is separated from the other electrodes, a DC arcing current flows between the electrodes, and in this instance, when a commutating current from the commutating circuit flowing in the opposite direction to the DC arcing current is superposed over the DC arcing current, the resultant current assumes an alternating current and the DC arcing current is interrupted at a current zero point of the resultant alternating current.
The present inventors noted that when the interruption of the arcing current using a commutating circuit is repeated in the above manner many times, a restriking between electrodes due to interruption failure sometimes occurs. The inventors investigated the causes thereof in many ways and found out the following facts.
Namely, after an interruption command is transmitted to the vacuum circuit breaker, the commutating switch closes with a predetermined delay from the command transmission. However, when the commutating switch closes before the insulation between the electrodes sufficiently recovers and a commutating current flows between the electrodes, the DC arcing current cannot be interrupted. This problem is caused by size errors of many mechanical parts such as levers, links, pins and shafts which constitute an actuating mechanism arrangement for the vacuum circuit breaker. The effect of the size errors in the actuating mechanism arrangement appears on the difference of separating speed of the electrodes, and when the separating speed is slow, the commutating switch closes before the insulation between the electrodes has been sufficiently recovered. As a result, a DC current interruption without fail could not be achieved, which condition is likely to escalate an accident.